1. Technical Field
The present invention relates to a constant-force device and a micrometer.
2. Related Art
In micrometers in which a spindle is displaced in an axial direction by rotating an operation sleeve to measure a dimension of an object to be measured based on the displaced position of the spindle, there is known a micrometer which includes a constant-force device which maintains a measuring force that is applied to an object to be measured from a spindle at a constant level (for example, refer to Patent Literature 1).
The constant-force device described in Patent Literature 1 is a unit that is disposed in an interior of an operation sleeve and which is adapted to cause the operation sleeve to turn idly relative to the spindle when the spindle is brought into abutment with the object to be measured whereby a load of a predetermined magnitude or larger is applied to the operation sleeve. Then, the constant-force device includes a shaft, a first ratchet wheel, a second ratchet wheel, and a compression spring.
The shaft extends along an axial direction of the spindle and is connected to the spindle at one end thereof, supporting rotatably the operation sleeve.
The first ratchet wheel is fixed to an inner circumferential surface of the operation sleeve and rotates relative to the shaft in response to the rotation of the operation sleeve. Additionally, a plurality of teeth are formed on the first ratchet wheel that project in an axial direction of the shaft and which are provided side by side or continuously along a rotational direction of the first ratchet wheel about the shaft.
The second ratchet wheel is mounted so as not to be allowed to rotate relative to the shaft but to move along the axial direction of the shaft. A plurality of teeth are formed on a surface of the second ratchet wheel which faces the first ratchet wheel for mesh engagement with the plurality of teeth of the first ratchet wheel.
The compression spring biases the second ratchet wheel towards the first ratchet wheel.